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You are here: Home / Newsletter Archive / July 2020 Newsletter: Density Anomalies and Geophysics – Concepts and Methods

July 2020 Newsletter:
Density Anomalies and Geophysics – Concepts and Methods

In general, the density of the earth increases with depth, from a lighter silicic crust (or basalt in the oceans) to a sequence of denser, more metallic rocks from the mantle to the inner core. This is what we expect when digging or drilling into the earth. But occasionally we are surprised by the discovery of lighter, less dense earth layers beneath denser ones. Sometimes these layers are actually occupied by nothing but air or water and they surprise us in the form of weak rock zones, sinkholes and voids, which can cause great mischief for us and our infrastructure projects including roadways, pipelines, buildings and other projects that depend on a solid earth foundation.

Image of sinkhole
Example of a collapsed sinkhole at the ground surface (also known as an auto eater)

Fortunately, modern surface geophysical techniques can be used to help identify areas where subsurface voids are located so that they can be avoided at the design phase of a project. The primary tools at the geophysicists’ disposal are electrical resistivity, seismic profiling, and ground penetrating radar. Each of these techniques has unique capabilities in identifying voids and weak rock zones.

If voids are located within several feet of the ground surface. Ground Penetrating Radar (GPR) can be an effective tool. The radar waves will show a significant reflection at the interface of the overlying denser and underlying less dense materials (or void). As depth increases (10’+) other geophysical techniques can be successfully applied. Using Electrical Resistivity Tomography (ERT), changes in density can be found by processing the apparent electrical resistivity at various depths (up to 100′) and locations. Using interpretive software the profile of underlying densities can be approximated.

GPR data collection
Looking for voids under concrete with Ground Penetrating Radar.
Resistivity data collection
ERT survey for large voids in Texas

 

 

 

 

 

 

 

MASW Data Collection
Seismic Data Collection for Subsurface Density Anomalies

Seismic data, involving the transmission of sonic waves through the subsurface, can also be processed with appropriate software to allow interpretation of underlying earth densities. There are certain instances where seismic is incapable of accurately imaging below density inversions but the technique can be effective to the point of the inversion. In the oil and gas industry, the use of seismic methods led to one of the major breakthroughs in the discovery of natural gas by revealing so called ‘bright spots’ in the data which typically highlight areas where less dense gas filled porous rocks underlay overlying denser materials.

Using the methods described above, when applied appropriately for the site conditions and objectives, can help architects, engineers, land planners and designers, geologists and construction professionals identify potential areas of poor subsurface conditions, saving significant time and money. Geophysical capabilities are continually improving through software and hardware improvements, with 3D and GPS advances making the acquisition and interpretation more accurate and understandable. Stay tuned.


Example of Shallow Sub-Slab Voids Identified in a GPR Survey (~3 Feet Below Ground Surface) 

GPR transect with void

 

Example of Electrical Resistivity Survey Identifying Karst/Sinkhole Conditions

ERT transect with karst

 

Example of Seismic Results Showing Density and Depth Variability in a Rock Formation

MASW Cross Section

On Topic:

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Filed Under: Newsletter Archive Tagged With: drilling hazard, Electrical Resistivity, Geophysics, GPR, ground penetrating radar, karst, MASW, oil field hazard, seismic, sinkhole, top of rock

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Recent Articles

  • October 2020 Newsletter:
    Supplemental Tools for Geophysics – How We Present the Results
  • July 2020 Newsletter:
    Density Anomalies and Geophysics – Concepts and Methods
  • January 2020 Newsletter:
    More interesting information about bedrock fractures and how to analyze them
  • October 2019 Newsletter:
    Seismic Geophysical Surveys to Examine Bedrock Depth and Integrity
  • June 2019 Newsletter:
    How the Roads that lead to Rome were made

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